The present invention relates to an induction motor which comprises a stator having a stator winding and a rotor coupled to the motor shaft and separated from the stator by an air gap. With the induction motor according to the present invention the power factor can be selected over wide adjustable ranges as desired.
Induction motors are among such electrical machines that have a stator having a stator winding, and a rotor which has a winding in which the rotor current becomes induced through the stator winding. The induction motors can have a very simple construction, having a cage-type rotor (shorted rotor), wherein the cage comprises rods and wherein the rods at the end of the rotor are coupled by a ring (shorting ring).
The efficiency of electric motors can be defined by the relationship of the useful output which can be taken off from their shaft, with respect to the power supplied from the network to the motor. In induction motors the efficiency depends on the voltage, the current and the phase angle. The effective output which can be taken off of the shaft depends on various factors. It is clear that the efficiency, by reducing the power which is taken out of the network, will improve the effective output if it remains at an unchanged or nearly unchanged value. A large portion of the received power is taken up by the VAR, which can be limited by appropriate and complex capacitor arrangements. The use of compensating capacitors for the VAR control means a worsening of the operating conditions and an increase of the manufacturing costs of induction motors. The VAR depends from the supply voltage and the supply current, as well as from the power factor, wherein the power factor represents the cosine of the phase angle difference appearing between the current and the voltage.
The improvement of the power factor has been a subject of many inventions. In addition to using capacitor units, there are different solutions known, such as the use of the hysteresis motor according to German Pat. No. 32 32 914. In that a rotor is proposed in the form of a hollow cylinder and made from a permanent magnet, which is subjected to a radial magnetization during the operation of the motor. By an appropriate selection of the thickness of the walls, one may attain that nearly all of the magnetic circuits become closed through the air gap of the motor, resulting in an improvement the power factor.
The disadvantage of the known solution resides in that the power factor can be adjusted only rather inaccurately; that is, for various motors having similar construction, different power factors will be present.
Similar disadvantages are associated with the reluctance generator illustrated in German Pat. No. 28 32 165. In this patent the use of a generator shaft made from permanent magnetic material is proposed. The rotor of the generator is coupled with this shaft, which also carries an element deflecting the magnetic flux. The magnetic flux of the exciter winding will be concentrated in the middle portion of the generator, whereby the lowest magnetic resistance is overcome, so that a element deflecting the flux and made from a ferromagnetic material can be placed occupy the middle portion of the exciter winding.
In the patent literature proposals can be found according to which in the inner space of a stator of a motor, several rotatable elements are arranged. However, as can be seen in the following, only special situations and with special motors can be solved with those proposals.
The provision of an additional rotor is, for example, known from German published application No. 30 45 820 (Siemens). Such proposal requires the provision of a cage rotor, as an outer rotor, on the motor shaft and its concentric arrangement with an inner rotor provided with a winding, wherein the outer rotor and the stator are connected to an outside voltage source. As a result a variable speed for the rotor can be set. A motor built according to the above is a special machine, wherein an increase in the power factor cannot be attained.
In application WO No. 85/03174 (Bosch) the subdivision of the cage into two portions is proposed, namely, for an outer cage and a freely running inner part made from a ferromagnetic material. As a result, a single phase motor is created having a small output and two or four poles, wherein the increase of the power factor is not intended and cannot be attained under practical operating conditions.